Ferromagnetic core



g- 26, 1 J. J. MOYNIHAN 2,849,696

FERROMAGNETIC CORE Filed Aug. 4, 1953 2 Sheets-Sheet 1 v 0 J5 35 M 54 i4 0 I/ZO INVENTOR. Z ii J07m/ Jflgymkaw g- 25, 1958 .J. J. MOYNIHAN 2,849,696

FERROMAGNETIC CORE Filed Aug. 4, 1953 2 Sheets-Sheet 2 RUL DEVICE CONT DEVI DEVICE DEVICE 1 0 J J I J03 INVENTOR.

tates Patent FERROMAGNETIC cons John J. Moynihan, Chicago, Ill., assignor to M & F Associates, St. Joseph, Mich, a limited partnership Application August 4, N53, Serial No. 372,213

3 Claims. (61. 336-212) This invention is concerned with ferromagnetic cores of the stamped lamination type such as are used in small transformers and the like, and with the coil windings thereon. I

Small ferromagnetic cores made of stamped laminations are used for small power transformers, audio transformers, chokes and the like. These articles frequently are used in portable apparatus where light weight is a necessity.

The magnet steel used in making the core laminations occasionally is in short supply, particularly in times of national emergency when large quantities of steel are diverted for military needs. The same is true of the copper used for core windings, and frequently to an even greater extent as domestic sources of copper are quite limited compared with domestic sources of iron and steel.

From a manufacturing standpoint in devices of this kind material costs are of prime importance, and it is a matter of great importance to keep these costs down as much as possible. Copper is more expensive than steel, and it therefore may be more desirable to reduce the quantity of copper used, even at the expense of requiring a slightly greater amount of steel. However, both are elements of importance, and are conserved by the present invention, in respect to the power rating; of the electrical unit provided.

Accordingly, it is an object of this invention to provide a ferromagnetic core of stamped laminations for magnets and the like utilizing a minimum of steel.

A further object of this invention is to provide a I stamped lamination core transformer or the like utilizing a minimum of copper in the coil winding or windings.

Yet another object of this invention is to provide a stamped lamination core transformer or the like wherein the total required steel and copper is reduced to a minimum.

This invention contemplates achieving the foregoing objects by the provision of a ferromagnetic core having a cylindrical center leg with a cylindrical coil wound thereon. The laminations comprising the core necessarily take several diflerent forms. If a plurality of dilferent dies, one for each different form of lamination, were used, and a different strip of ferromagnetic sheet stock were fed to each die, certain disadvantages would result.

The strip stock from which ferromagnetic cores are made is subject to varaition from one point to another in the strip. This variation involves not only the physical characteristics (such as the gage) of the material, but also the chemical composition and the electrical and mechanical properties. Ferromagnetic strips or sheets in commercial production constitute a continuous spectrum of quality. Steel mills aim for the highest quality consistent with practical expense, but there are inevitably many occasions when the highest desirable quality ferromagnetic material is not produced. The result is that even in a single strip of ferromagnetic material, which may run hundreds of feet in length, there are cyclic drifts in the quality and magnetic properties of the material. With a, such a cyclic drift, the material of any given strip may range from excellent to poor in magnetic qualities. Again all of the material of any given strip may be acceptable (although of varying grades), while all of the material of another strip may be unacceptable.

If all of the laminations of a given configuration were to be produced by a single die from a single strip, and that strip should turn out to have a high percentage of ferromagnetic material of undesirable or unacceptable magnetic properties, then all of a large number of cores might be unacceptable, or at least of a lower grade than desired. It is possible that of the cores of a given batch could be below standard, with only 4% of the material from which they were made being defective.

It is an object of this invention to produce laminated ferromagnetic cores having cylindrical center legs, wherein the magnetic properties of any given section of strip stock are concentrated in a relatively few cores, thereby allowing the cores to be graded commercially without the production of a large number of unacceptable or low grade cores.

More specifically, it is an object of this invention to produce laminated ferromagnetic cores having cylindrical center legs, wherein all of the laminations of a given core are formed from successive increments of a strip of sheet stock.

Other objects and advantages of the present invention will be apparent from the following description when taken in connection with accompanying drawing wherein:

Fig. 1 is a perspective, partially exploded view of a transformer constructed in accordance with the principles of my invention;

Fig. 2 is a perspective view of the main portion of the core of the transformer;

Fig. 3 is an end view of a part of the core with the coil windings thereon taken substantially along the line 33 of Fig. 4-;

Fig. 4 is a cross-sectional view through the transformer taken substantially along the line 4-4 of Fig. 3;

Fig. 5 is a plan view illustrating the formation of the laminations of the transformer from a sheet steel blank;

Fig. 6 is a view of the E-shaped laminations illustrating the necessary shaving of the windows in the outer laminations;

Fig. 7 is a perspective view of the finished transformer;

Fig. 8 is a perspective view of one form of apparatus for producing my cores; and

Fig. 9 is a horizontal sectional view thereof.

Cores for small power and audio transformers, chokes, and the like generally are made by stamping laminations from sheet steel and stacking them together. The cores are E-shaped with an I-shaped bar lying across the ends of the legs of the E and parallel to the back so that the resulting core is symmetrical. The I bar contacts the ends of the legs of the E and is mechanically connected thereto. To conserve material in manufacture, the E- shaped laminations are blanked out in pairs with the ends of their legs contacting one another. The material stamped out from between the legs, and the gaps so formed forming the windows in the transformer core, thus serve as the laminations forming the I bar of each core. No material is Wasted except for that stamped out for holes or guides in the punch press and often later serving as bolt holes for holding the laminations together or for mounting the cores.

Heretofore all of the laminations going into One magnetic core for a small transformer, choke or the like have been made identical so that the central leg of the E-shaped core portion is square or rectangular in cross section. The other two legs, the back, and the I bar conventionally have a rectangular .cross section equal to one half the cross section of the central leg. This maintains the flux density, and hence the core losses, uniform throughout the core and local overheating is avoided. The nominal size of such lamination, transformers being used herein for illustration, is signified by the dimension across the central leg of the E-shaped lamination, e. g. one inch.

As disclosed herein, both magnet steel for the core and copper for the coil windings are conserved 'by the provision of a core wherein the central leg of the E is substantially cylindrical. The core is made up of laminations wherein the lamination portions forming the central leg of the E decrease progressively in width with each lamination from the center of the core out toward the edges thereof. The lamination portions forming the other two legs of the core increase in width progressively from the center out toward the edges of the core so that the coil gutter remains substantially constant in area. Actually, the window area must increase slightly in working from the center out toward the edges of the core as the chordal portions of the cylindrically wound coil lying in the planes of the various laminations increase in length moving outwardly from the center.

Referring now more particularly to the drawings, there may be seen in Figs. 1-4 the parts comprising a small power transformer constructed in accordance with the principles of my invention. A magnetic core may be seen to be comprised of I bars or elements 12 and E- shaped elements 14 with the I bars arranged against the ends of the legs of the E so as to form a closed magnetic circuit therethrough.

The E-shaped elements 14 are in the form of individual laminae 16 each in the form of a plane sheet of ferromagnetic material, generally transformer steel, having the configuration of a block letter E. Many different types of transformer steel are available commercially, and the type to be used, as Well as the thickness of the laminae, will be determined by the use to which the core is to be put.

In general, the width of the backbone of the E will be constant from one lamination to another. This width is indicated in Fig. 4 by the dimension C. The width of the legs of the E will be dissimilar from lamination to lamination, although the laminae will be generally symmetrical about a line perpendicular to the backbone of the E and lying along the central axis of the middle leg of the E. The width of the outer legs of the E is indicated at I in Fig. 3, While the width of the middle leg of the E-shaped laminations is indicated at K. The width K will be seen to decrease while the width J increases in successive laminations outwardly from the center of the core.

The I bars 12 are also laminar in structure, and in general the laminations are identical. The width of the I laminations is indicated in Fig. 4 at D, while the length is indicated at M. The width D of the I laminations generally will be equal to or less than the minimum window with N as indicated in Fig. 3. The length M of the I laminations must be equal to or less than twice the window depth H (Fig. 4) if the so called scrapless E and l lamination stampings are to be made. The windows are identified by the numeral 18 and consist of the space of an individual lamination between the side legs 20 and center leg 22 of the E, and limited at the ends by the backbone 24 of the E and a line extending across the free ends of the E legs.

When a group of laminations is stacked together to form a core, the windows in the laminations form openings 26 through the stack. These openings exist to accommodate a coil to be placed over the middle leg of the E and hereinafter will be termed coil gutters in contra-distinction to the openings in the individual laminations which hereinafter will be termed windows.

In conventional laminated magnetic cores for small power transformers and the like, the dimensions of the windows of the laminations all are identical and are identical with the dimensions of the coil gutter. According to the principles of my invention, I provide laminations wherein the windows diflfer in size from lamination to lamination as will be apparent immediately hereinafter.

The width K of the center legg 22 of the E decreases from lamination to lamination so that when the laminations are stacked together, the center leg is substantially in the form of a right circular cylinder as will be seen in Figs. l3, said cylinder having a diameter A. The diameter A of the center leg 22 is a function of the cross-sectional area of magnet steel desired in the center leg and is also the function of the stack height B. The width of the backbone C of the E element of the core is also a function of the stack height'B, since the product C B of the backbone section must be equal to one-half the area of the cross section of the center leg for uniform flux density throughout the core.

A cylindrical coil 28 is wound on the center leg 22 of the E element of the core. More correctly, the coil is wound on a winding machine, and then is slipped over the center leg. The coil, in the case of the transformer herein illustrated, will consist of two windings, the leads of which have been omitted for clarity of illustration.

The power rating of a transformer depends on the cross-sectional area of the core and on the ampere-turns in the coil, and the latter is a function of the coil gutter cross-sectional area, i. e. F H, where F is the width of the coil gutter.

It will be seen in Fig. 3 that the outer edge of the coil gutter comprises a cylindrical surface of greater radius than the cylinder forming the center leg 22. Therefore, the dimensions K decrease more rapidly than the dimensions I increase when working out from the center lamination of the core. For uniform flux density, the sum of the cross-sectional areas of the two outer legs of the core should be equal to the cross-sectional area of the middle leg of the core. In other words, the sum of all of the Js should equal the sum of all of the Ks.

Also, for uniform flux density throughout the magnetic circuit, the width D of the I bars 12 should be equal to the width C of the backbone of the E element 14, and the stack height of the I bars should be equal to B, the stack height of the E elements. The various core dimensions therefore must be in a certain proportion to one another to maintain uniform flux density throughout the magnetic circuit.

For most efiicient utilization of magnet steel, the core must be as nearly as possible scrapless, i. e. the I bars must be produced from the stock removed in the stamping out of the windows of the E elements. In terms of the letter dimensions heretofore enumerated, with the addition of width G of any window 18, each of the Gs must be equal to or greater than D, and M must be equal to or less than 2H. Limitations are imposed on the rating and use of electromagnetic equipment such as the core herein described by the heat developed. Sources of heat within a transformer core are two in number. There is a power loss in the core caused by eddy currents, and there are hysteresis losses. There is also a power loss in the windings due to the passage of current through a finite resistance. Flux density for cores of the type herein under consideration but with the conventional square center leg type have been pretty well standardized through years of experimentation and commercial use. The standard flux densities are maintained in my cores. Therefore, if my new core has the same flux density and is equal to or less in weight than the old square legged, rectangular stacked core, the core losses in my core will not exceed those in the old core.

Taking equivalent transformers constructed according to the principles of my invention and of the old square legged, rectangular stack core type, the two transformers must have the same cross-sectional area and an equal number of winding turns. The length of conductor required to enclose a given cross-sectional area is at a minimum when the enclosed area is in the form of a circle. The resistance of a coil wound from the same size of wire as used in a conventional transformer will be less in the case of the circular or cylindrical middle leg core as herein disclosed than in the case of the square legged, rectangular stack core. The cylindrical coil in my transformer therefore would run cooler if wound from the same size wire. Therefore, a smaller conductor having a higher resistance may be used to tend to bring the PR loss up to that permitted in conventional transformers. The smaller conductor obviously reduces the amount of copper needed to attain the same number of turns. Furthermore, the smaller conductor size allows more turns to be wound per layer in the same coil length, and this in turn reduces the number of layers required to get the total number of turns. Since the number of layers is reduced, and the wire itself is of smaller diam eter, the mean length of turn in the coil is reduced. The reduced mean length of turn further shortens the total length of wire for the transformer windings, and this tends further to drop the resistance and the PR loss.

Due to its contour or shaping, a round coil turn is about 9% less in length than 'a square coil turn embracing the same cross-sectional core area. Thus the round coil turn requires 9% less, by weight, of copper; and inasmuch as resistance varies directly with length, the round coil turn has approximately 9% less resistance than an equivalent square coil turn. If it is desired only to maintain the core resistance within certain maximum limits, to thereby limit power and heat loss, advantage may be taken of the foregoing, in my invention, to reduce the wire size, to thereby effect a further saving, by weight, in the copper required. A reduction in wire size of one-half size or gage increases the resistance, for any given length, approximately 12%. Considering that with my invention a turn length of 9% less is employed, there would result, with a decrease in wire size of onehalf size or gage, an apparent or anticipated resistance increase of 3%. However, the use of smaller wire permits more turns per layer in the core, and accordingly fewer layers for the same total number of turns, as compared with a conventionally shaped and constituted core. The thinner layers, and fewer number of layers, with my construction, reduces the mean diameter of the core so that for a given total number of turns (as compared with conventional practice) less over-all length of wire is required, resulting in a resistance decrease due to this factor of approximately 3%; so that the final resistance value of the completed or composite core will be approximately the same as in the conventional construction. However, the combination of the foregoing factors referred to, including the permissible use of smaller wire, and the decreased length thereof for equivalent performance, results in a saving of copper, by weight, as compared with a conventional coil, of approximately 26%.

In conventional square turn coil winding, there is .an alternate increase and slackening of tension as the wire is wound around the corners of a mandrel and then passes the flats. This causes the square coil to bow outwardly on the straight" edges. The increased tension at the corners of a square coil also causes the wire to be stressed in tension and thus become reduced in cross section, as indicated by the higher values of electrical resistance for a square coil as compared with the resistance of the wire before the coil is formed. Consequently square wound coils can normally in practice use only 85% build (the radial thickness of the coil relative to the corresponding dimension of the core window), whereas a 90% build can be used with my cylindrically Wound coil.

The bowing of the straight sides of a square turn coil prevents its being placed snugly in contact with the square center leg of a conventional transformer, whereas my cylindrically wound coil can be placed substantially in contact with the cylindrical center leg at all y points.

According to the principles of my invention, there would be a theoretical saving of 26% in weight of copper. Due to the practical consideration of winding the coil from smaller wire which has a higher cost per pound this weight saving results in an actual reduction of about 20% in cost of the copper used. Although a saving in copper is thus engendered, it might appear that extra steel would be required to manufacture the core due to the non-uniform width of window, but such is not the case as will presently be shown.

As illustrated in Fig. 5, the E laminations are stamped out of strip stock in pairs with the legs confronting one another. The I bars thus are formed from the material stamped out to form the windows 18. Circular sections are punched out as at 29 for pilots in a punch press, and may serve as bolt holes for securing the laminations in stacked relation or for mounting the magnetic cores. iJ-shaped sections 30 also may be punched out of the E backbones 24- and out of the I bars 12 as illustrated in Figs. 4 and 5 for securing the laminations in stacked relation or for mounting the finished magnetic core.

For maximum utilization of magnet steel, the width D of the I bars is made equal to the minimum window width N. As shown in Fig. 6 this leaves a greater or lesser amount of waste material to be shaved off of the E between the general window width G and the minimum Window width N, N being equal to D. In the specific case of the outside laminations having the minimum center leg width K, the amount to be shaved off, as enclosed by the dashed lines, is considerable. This amount is all waste. This waste material makes it appear that a greater amount of steel would be necessary to manufacture one of my cylindrical core transformers than would be necessary with a conventional square legged, rectangular stack core transformer, the ratings of the transformers being equal. However, as previously stated, there is in fact an actual core metal saving.

The finished transformer as shown in Fig. 7 may have the laminations held in stacked relation and may have the I bar held against the ends of the E legs by a sheet metal bracket 32 in the form of an elongated metal strip extending along three edges of the transformers. Flanges 34 on the edges of the strip clamp the laminations together, while apertured, extending ends 36 on the strip serve for mounting the transformer. Tabs 38 on the lower ends of the vertical ends of the flanges 34 clamp the I bar 12 against the E section I4.

Contrary to the apparently greater amount of steel necessary to manufacture my transformer, I have found that a saving in steel actually can be effected if the dimensions are chosen properly. I have found experimentally that in a cylindrical core one inch equivalent transformer (i. e. one having the same power rating as a conventional square center leg, rectangular stack transformer having a center leg one inch square) when the width of the backbone of the E, and also the width of the I, varies between .4577-.4585 in., there is a 1% savings in the gross amount of steel required over that required for a conventional transformer. Within this range, the total volume of the transformer core varies little while the coil gutter width varies considerably.

Outside of the aforementioned values, there may still be a net savings in copper and magnet steel. That is to say, a sufiicient amount of copper can be saved to ofifset any increase in the amount of steel required. This may in some instances be all that is required, as where the steel costs are minor in relation to the cost of the copper. However, the invention in the optimum arrangement of the parts, as set forth herein, effects an actual saving in both materials.

The space factor of the coils used in making transformer windings is not constant as there is no constant relation between the thickness of the interlayer paper insulation and the wire diameter. Consequently, it is not possible to establish any definite figures wherein the quantitive savings in copper equals or offsets the quantitive increasein steel. When considering the net savings from a cost standpoint, still another variable enters in as there is no constant ratio between the prices of copper and steel.

There is a critical ratio for the width of the backbone of the E in any size cylindrical core transformer constructed according to the principles of my invention which requires a minimum amount of steel. This critical ratio is: thickness of the backbone of the E=.4577 width of middle leg of equivalent square core transformer. In a conventional square core transformer, this ratio is .5.

Various conditions earlier have been set up regarding the interrelation of the sizes of various components of a transformer constructed in accordance with the principles of my invention. Certain of the dimensions of these components are variables, notably the window width, the width of the center leg of the E, and the width of the end legs of the The following table supplements and summarizes the desirable relative dimensions of the components of the transformer core. In this table, the unit of length is X, and is equal to the square root of the crosssectional area of the middle leg.

Diameter, A: 1.1488X Stack height, B=l.0924X Backbone of E, C=.4577X Width of I bar, D=.4577X Coil gutter width, F =.4577X Window Width, G varies from .4577X to .69796X Window height, H 1.44199X Width of end legs, J varies from .40989X to .80652X Width of center legs, K varies from .35554X to 1.1488X Length of I bar, M=2.88398X Minimum Window width, N =.45'77X Maximum possible coil diameter, Q=2.0642X In my copending application Serial No. 372,367, now Patent No. 2,788,210, filed of even date herewith and entitled Apparatus and Method for Making Laminated Ferromagnetic Cores, l have fully set forth apparatus for and methods of producing the cores disclosed herein. Reference may be had to this copending application for a full and complete disclosure of such apparatus and methods. One suitable apparatus for producing such cores is shown in Figs. 8 and 9. In these figures, the E-shaped laminations are modified slightly in the punching of the pilot holes as will be obvious from an inspection of the drawings, but this modification in no Way affects the principles of the invention.

The apparatus includes a punch press (not shown) of conventional construction. A standard 0. B. I. (open back incline) type press may be used, but I prefer to use the type known commercially as 21 Henry and Wright due to the more accurate alignment of punches and dies. The apparatus further includes a subpress 40 comprising a punch holder and a die shoe. The punch holder and die shoe will be mounted on the head and bolster of the punch press as will be understood. Guide rods 46 extend between the punch holder and die shoe to insure proper alignment thereof at all times.

A strip of stoac is indicated at 48, and it Will be understood that this strip is fed intermittently in timed relation with the reciprocation of the punch press by any suitable means. The subpress '30 is provided at its entering end With a supporting die for supporting the strip stock 43. A strip guide 52 is positioned above the die 50 and is channel-shaped in configuration to guide the strip laterally. The guide 52 is provided with a pair of apertures 54 in its upper surface providing passage for a pairof cylindrical punches as for punching pilot holes 58 in the strip of stock. The slugs stamped from the apertures 58 are utilized for controlling the lateral position of the punch and dies by means of suitable apparatus disclosed in my U aforesaid copending application, the details of such apparatus not being important here. The apertures 58 cooperate with pilots 60 for properly positioning the strip throughout the ensuing punching operations.

A punch carrier 62 is mounted on the punch holder 42 for transverse movement relative thereto by means of a dovetail slide 64- fitting in a dovetail slot 66 in the under side of the punch holder. The punch carrier 62 carries a rectangular punch 60. A cooperating movable die carrier 70 is mounted on the die shoe 4 for movement transversely thereof by means of a dovetail slide 72 on the die carrier and a dovetail groove or guideway (not shown) in the die shoe. The punch carrier and die carrier are joined together for movement transversely of the subpress 40 by means of guide rods 74 fixed in one of the carriers, for instance the die carrier, and reciprocable through complementary apertures 76 in the cooperating carrier. The punch carrier 62 and die carrier 7'0 along with the guide rods 7 form a movable subpress tool 78.

The die carrier 70 carries a die 30 having a rectangular aperture therein aligned with the punch 68 for punching rectangular shaped I bars from the strip 48. The movable subpress tool 78 is connected by means such as a connect ing rod 82 to a control device 84 for shifting the subpress tool 78 transversely of the subpress in accordance with a predetermined pattern and in accordance with the thickness of the strip stock 40. The punch 68 forms rectangular apertures 86 in the strip stock 48.

A second subpress tool 88 similar in construction to the subpress tool 73 and having a cooperating rectangular punch 90 and rectangular die aperture (not shown) is mounted for movement transversely of the subpress 40. The mounting is generally similar to that heretofore described and includes dovetail slides 92 fitting in cooperating dovetail guideways 94 in the carriers 42 and 44. The subpress tool 30 is Controlled in lateral position relative to the strip 48 by a connecting rod 96 and a control device 98. The rectangular punch 90 stamps another I bar from the strip and leaves a second rectangular aperture 100 which is of the same size as the rectangular aperture 86 and Which is symmetrical about the center line of the strip 4-8 with the aperture 86.

A third subpress tool 102 substantially identical with the tool 78 is mounted in the subpress 40 adjacent the tool 88. The subpress tool 102 includes a punch carrier 104 and a die carrier 106 mounted by means of cooperating dovetail slides and guideways 108. The punch carrier 104 carries a rectangular punch 110 aligned with a rectangular aperture 112 in a die 114- carried by the die carrier 106. Rods 16 maintain the punch carrier and die carrier in properly aligned relation. A control device 118 similar to those previously mentioned operates through a connecting rod 120 to shave a section of metal from the stock to enlarge each aperture 36 in accordance with a predetermined pattern and in accordance with the thickness of the strip stock.

Yet another subpress tool 122 similar to those previously described is similarly mounted by means of dovetail slides and guideways and includes a punch carrier 124 (Fig. 8) carrying a punch 126 (Fig. 9). It Will be understood that there will be also a cooperating rectangular aperture in a die carried by an underlying die carrier. A control device 128 acts through a connecting rod 130 to shift the subpress tool 122 for shaving and enlarging each of the rectangular apertures 100 so that the apertures 86 and 100 are of the same size and are symmetric about the center of the strip 48 at any given transverse section of the strip.

A parting punch 132 is carried by'the punch holder 42 near the discharge end thereof and cooperates with a rectangular opening in a parting die carried by the die shoe 44 for parting the strip into pairs of E-shaped laminations.

Suitable stripper plates of any known designs are incorporated in the various subpresses. Any suitable means may be used for stacking the E-shaped laminations coming from the punch press, but stacking chutes of conventional design preferably are utilized for automatically stacking both the E-shaped and the I-shaped laminations. In any event, alternate sections of the strip are stacked adjacent one another in forming my ferromagnetic cores so that the composition of the laminations in any given core is substantially uniform. Accordingly, an inferior or substandard section of ferromagnetic material will affect only a very few cores, which can be rejected or commercially down graded, instead of a large number of cores.

I have found that excellent results in finished cores can be obtained if at least /3 of the adjacent laminations in a core .are derived from portions of a continuous strip of ferromagnetic material which are spaced no farther than five times the increment of strip length required to produce a single lamination.

Various modifications of the specific example shown and described are possible and come within the scope of this invention insofar as they fall within the spirit and scope of the appended claims.

I claim:

1. A ferromagnetic core comprising a. member substantially in the form of a block letter E and having a portion connecting the ends of the legs of the E, said portion being parallel to the backbone of the E, the center leg of the E being substantially cylindrical, the opposite faces of the member including the legs and backbone of the E being planar, said member having a predetermined height between the planar faces, the backbone of the E having a width in the direction of the legs and a length perpendicular to the direction of the legs and parallel to said planar faces, the portion interconnecting the ends of the legs having a width in the direction of the legs and a length parallel to the backbone of the E and equal to the length of said backbone, said legs having a length perpendicular to the backbone of the E, the backbone of the E and the portion connecting the ends of the legs being rectangular in cross section, the end legs of the E being rectangular in cross section except for the faces confronting the center leg which are substantially surfaces of a cylinder of predetermined diameter, the heretofore mentioned dimensions occurring in substantially the following proportions, X being the unit of length: diameter of center leg=l.l488X; predetermined cylinder diameter=2.0642X; height between planar faces=1.0924X; width of backbone of E=.4577X; width of portion interconnecting ends of legs=.4577X; length of legs:l.44l99X; and length of backbone of E=2.88398X.

-2. A ferromagnetic core as set forth in claim 1 wherein the core is made up of stacked E-shaped laminations and the ends of the legs of the E-shaped laminations are connected by I-shaped laminations, the center legs of the E-shaped laminations decreasing in thickness from the center of the core out to the substantially planar faces, and the width of the end legs increasing in thickness from the center of the core out toward the planar faces, the thickness of the center leg laminations varying approximately from .3554X to 1.1488X, and the thickness of the end legs varying approximately from .40989X to .80652X.

3. An electromagnetic device as set forth in claim 2 and further including a cylindrical coil on the center leg, and means for holding the laminations stacked together.

References Cited in the file of this patent UNITED STATES PATENTS Bellman et al. June 7, 1898 Warsher June 13, 1944 High-Q Iron-Cored Inductor Calculations, Javna; Electronics, August 1945, pp. 119-123.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,849,696 August 26, 1958 J ohn J Moynihan It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column '7, line 34; and column 10, line 25, for M80652)? read 56632X column '7, line 39, for "Serial No, 372,367, now Patent No, 2,788,210 read Serial No. 372,363

Signed and sealed this 31st day of March 1959.,

(SEAL) Attest:

KARL Ho AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,849,696 August 26, 1958 John J Moynihan It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column '7, line 34, and column 10, line 25, for "3065235" read ,56632X column '7, line 39, for "Serial No, 372,367 now Patent No 2,788,210" read e- Serial No 372,363

Signed and sealed this 31st day of March 1959 (SEAL) Attest:

KARL Ho AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

